The present application involves a comprehensive investigation of the basic enzymological and spectroscopic properties of four related mononuclear molybdenum enzymes: xanthine oxidoreductase, sulfite oxidase, DMSO reductase and arsenite reductase. This group includes representatives of all three major families of mononuclear molybdenum enzymes. The overall goal of the proposed work is to gain a more complete understanding of fundamental aspects of the mechanism of action of each of these enzymes by comparing and contrasting their physical, chemical and electronic structures. A comparison among enzymes type with regard to the relationship of structure to function will also be made. To this end a series of rapid reaction kinetic, spectroscopic, theoretical and molecular biological studies will be undertaken that build directly on past work in the PI's laboratory. The proposed work is intended to integrate both mechanistic and spectroscopic studies of these enzymes with their known crystal structures and to elucidate underlying themes in reactivity by comparing and contrasting their properties. This approach that has proven extremely successful in studies of the generally much better understood biological systems that possess heme and flavin prosthetic groups.